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Wall stack up opinions – using rigid foam as air barrier between double stud wall?

rocket190 | Posted in Energy Efficiency and Durability on


I’m in climate zone 6A looking to build a partition wall between an attic bonus room. One side of the bonus room will remain as cold storage, and the other side will be heated by minisplits and resistance backup. I’m looking at building a double 2×4 wall separated by rigid foam. The exterior wall would be framed 24″ o.c. to maximize insulation. There will be no plumbing or wires in the exterior wall. The interior framed 2×4 wall will be framed 16″ o.c. and will contain wiring, and will possibly be plumbed for a sink.

From cold to warm, here is what I’m proposing:

1. 5/8″ fire rated gypsum
2. R-15 high density batt insulation installed in 2×4 wall framed 24″ o.c.
3. 1″ polyiso insulation detailed as air barrier
3. R-15 batt insulation installed in 2×4 wall framed 16″ o.c.
4. 1/2″ gypsum

The foam “gap” at the top of the wall would be covered with gypsum or suitable fire barrier.

As this is a non-structural wall and utilizes standard dimension framing, I thought that batt insulation with the foam would result in better performance than using something like taped plywood for the air barrier. Just want to make sure there aren’t issues with the foam. Depending on what foam I use, the wall would be vapor open both ways, but I want to make sure it doesn’t become a condensing surface.

I’ve read the articles on calculating the min thickness of exterior foam, but what rules apply when the foam is sandwiched? If this works it seems like a great wall design, as the assembly should come in at approx R-26 at a wall thickness of about 9″.

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  1. GBA Editor
    Martin Holladay | | #1

    I see no reason why your proposed wall won't work. Other readers may propose different wall solutions -- there is no end to the variations that are possible with wall construction. But if you like this stack-up, go ahead -- it will work.

  2. rocket190 | | #2

    Great to hear.

    Do you think that foil faced polyiso is a good option for the foam layer, or would you stick with something that is still somewhat permeable like xps?

  3. GBA Editor
    Martin Holladay | | #3

    If you want to create an air barrier, foil-faced polyiso is the easiest foil to tape.

    With all types of rigid foam, including polyiso, there are historical reports of foam shrinkage (and in a few cases, foam expansion). Some builders find these reports worrisome, and have concluded that taped rigid foam isn't dimensionally stable enough to make a good long-term air barrier.

    Others don't worry. Most rigid foam manufacturers claim that they have solved the foam shrinkage problem.

  4. rocket190 | | #4

    Good to note.

    I've used taped foam as air barrier and combination WRB on a few projects and so far so good. Taped joints are about 5 years old and still look great.

    As an aside, I'm glad I have the time to work on my projects when time allows. It's been about a year since my building was framed, and due to drying, the framing lumber has shrunk dramatically. The joints of the treated sills have opened to 1/4" gaps in spots. All the anchor bolts have been re-tightened twice, and it's time to do it again. I waited till now for my interior caulking/air sealing with the hopes that the bulk of the shrinking has already occurred.

  5. charlie_sullivan | | #5

    Martin said your proposed wall will work. As usual, he's right.

    Martin also said readers would propose alternatives. To make sure he's right about that, here's one:

    In place of the polyiso, use a sheet of the dreaded polyethylene. It's OK because it's in the middle of a wall that can still dry to either side. Then blow in cellulose and fill the space where the polyiso was going to be. Pretty much the same performance but cheaper. Unless you are doing the work yourself and not counting your time in the cost, in which case hiring out the cellulose work might start to look expensive compared to installing batts yourself.

  6. rocket190 | | #6

    Charlie, thanks for the comments. I might have to do blown in since I'm having a hard time finding high performance batts that fit 24" o.c stud spacing. Does anyone know if it would lead to a higher R stackup if I go with 16" o.c. spacing with r15 batts or 24" o.c spacing with r-19 compressed?

  7. GBA Editor
    Martin Holladay | | #7

    R-19 compressed into 2x4 stud bays, 24 inches o.c., beats the performance of R-15 batts in 2x4 stud bays, 16 inches o.c.

  8. Dana1 | | #8

    R19 batts compressed to 3.5" is exactly R13, but the framing fraction for a non-structural studwall 24" o.c. with single top plates and only single headers around windows etc is between 15-20% compared to 25% for a structural 2x4 studwall with doubled top plates a doubled headers over window, jack-studs, etc. (If R13s for 24" o.c. spacing are cheaper than R19s or conversely, go with the cheaper option. Compressed to 3.5" the performance & density is the same.)

    For a non-structural 2x4 studwall at 16" o.c. spacing you'd have to figure out the actual framing fractions of the two options and compare. R13 @ 24" o.c. vs. R15 @ 16" o.c. might be pretty close for the non-structural case. For instance, counting just the framing & insulation layer (not the wallboard etc):

    R13 at a 17% framing fraction comes in at R9.56, whereas R15 at a 22% framing fraction comes in at R9.60.

    R13 at a 15% framing fraction comes in at R9.87, whereas R15 at a 20% framing fraction runs R9.92.

    It would be difficult to get the framing fraction below 15% for the 24" o.c. wall, but it may be possible to duck under 20% for a non-structural 16" o.c. wall, in which case it would edge out the compressed R19s on performance, but at an uptick in cost.

  9. rocket190 | | #9

    Dana I will get the framing fraction down to about 10% on the non structural wall (no windows, no fireblocking, single plates top and bottom)

    Do you have a program that calculates these r values?

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